Transparent Text In Cycles Not Working
This is because the text is in the same place as the background plane.

TL;DR: Move the text a little above the plane so they don't intersect and interfere.

Blender has no way of knowing if the text should be on top (and invisible) or if the background plane should be on top (obscuring the text). Even though either scenario comes to the same result in this particular case, such an undefined situation is liable to cause undefined behavior.

What happens is some cycles form of z-fighting (the classic z-fighting artifacts only seem to appear when in ortho view and from some angles). If you put an object behind the plane and text, the issue becomes clearer: